code review, misc tweaks, cleanup

[torkus]

• addon, removes 'group-addon-count'
• toc, removes custom slugify.
• nfo, tweaks.

• [x] yagni pass
• [x] move ui/* into src/strongbox
• [x] merge db.clj into core.clj
• [x] review toc.clj
• [x] review nfo.clj
• [x] review addon.clj
• [ ] review core.clj
• [ ] review cli.clj
• [ ] review jfx.clj
• [ ] remove support for v1 imports (and exports?)
• [ ] ...
• [ ] review
• [ ] update CHANGELOG